Noncontact communication medium and noncontact communication system

ABSTRACT

An output of a resonant circuit  60  which electromagnetically couples with an external communications apparatus is rectified and taken out, and a resultant output voltage V is monitored by a switch control circuit  61.  After the output voltage V has risen to or beyond a threshold Vth1, a switch element SW is turned on and off, and a state of the switch element SW in which the output voltage V becomes higher is determined. While holding the switch element SW in the state thus determined, a power supply switch element PSW is turned on when the output voltage V has further risen. As a result, even when a plurality of noncontact IC cards  6  overlaid with one another are brought close to the communications apparatus, the individual noncontact IC cards  6  can carry out data communication.

TECHNICAL FIELD

[0001] The present invention relates to noncontact communications mediaused for noncontact communication. A noncontact communications mediumcalled a smart card, for instance, communicates identificationinformation whose typical examples include an identification number andinformation about an individual's physical features for authenticatingthe individual, value information whose typical examples include moneyinformation, and credit information used for credit cards, stores thesepieces of information, and modifies previously stored information basedon those pieces of information. The invention relates also to noncontactintegrated circuit (IC) cards which are card-shaped noncontactcommunications media and to data carriers attached to a human, a vehicleor a baggage when in use, for example. Furthermore, the invention isconcerned with noncontact communications systems in which a noncontactcommunications medium communicates with an identification apparatus suchas a personal authentication apparatus, an automatic ticket checker, anautomated teller machine which handles cash or electronic cash, or acharge collecting machine.

BACKGROUND ART

[0002] A noncontact IC card is provided with a resonant circuit. When anantenna coil which is part of the resonant circuit is moved to cross amagnetic flux generated from an information read/write device, aninduced electromotive force occurs from the antenna coil and theresonant circuit produces a high voltage. When the noncontact IC card isbrought close to an antenna section of the information read/write deviceand caused to receive electromagnetic waves from the antenna section,the resonant circuit is brought to a state of resonance. Datacommunication is carried out between the information read/write deviceand the noncontact IC card using the state of resonance. Also, thevoltage produced as a result of resonance of the resonant circuit isused to supply electric power to other circuits causing those circuitsto operate.

[0003] Let us now consider actual conditions of use of the noncontact ICcard. Since noncontact IC cards used as railway commuter passes, cashcards used at banks and credit cards used for shopping, for example, areso thin that a plurality of noncontact IC cards of this kind are heldoverlaid in a wallet or a pass holder and brought close to an antennasection of an information read/write device in many applications.

[0004] Since two or more noncontact IC cards are overlaid with eachother when in use in this case, resonant frequencies of the individualnoncontact IC cards deviate from carrier frequency of a signaltransmitted from the information read/write device due to mutualinductance between the noncontact IC cards. As a result, there can arisea case where the voltage produced in the resonant circuit drops andelectric power sufficient for signal reception is not supplied tointernal circuits of the noncontact IC card in use, leading to aninability to perform communication between the information read/writedevice and the noncontact IC card.

[0005] This phenomenon is now explained using FIG. 17. The horizontalaxis of a graph of FIG. 17 represents the frequency of a resonantcircuit of a noncontact IC card. The vertical axis indicates theabsolute value of a receiving voltage occurring in the resonant circuit.This is similarly applied to the horizontal axis and the vertical axisof a later-described graph of FIG. 18.

[0006]FIG. 17(A) shows a relationship between frequency and receivingvoltage that occur when one noncontact IC card is brought close to aninformation read/write device. In this case, the noncontact IC card isused alone. For this reason, resonant frequency f0 of the resonantcircuit matches carrier frequency fc of a carrier signal transmittedfrom the information read/write device, and a high receiving voltageoccurs at that frequency.

[0007]FIG. 17(B) shows a relationship between frequency and receivingvoltage that occur when two noncontact IC cards are overlaid with eachother and brought close to the information read/write device. In thiscase, the two noncontact IC cards are overlaid and are subject to theinfluence of mutual inductance between the two noncontact IC cards, sothat the resonant frequency of the resonant circuit varies and, asdepicted in the Figure, the resonant frequency f0′ deviates from thecarrier frequency fc of the carrier signal from the informationread/write device. For this reason, a low receiving voltage occurs atthe carrier frequency fc as compared to the receiving voltage at theresonant frequency f0′. As a result, there arises a problem thatsufficient electric power is not supplied to internal circuits of thenoncontact IC card in use, leading to an inability to performcommunication between reader/writer and the noncontact IC card.

[0008] To solve the aforementioned problems, it has been proposed topreset the resonant frequency of the noncontact IC card such that itdeviates to a higher frequency as compared to the carrier frequency.According to this method, the resonant frequency does not deviate somuch from the carrier frequency even if the resonant frequency decreaseswhen two noncontact IC cards are overlaid with each other and,therefore, it is possible to supply sufficient electric power to theinternal circuits of the noncontact IC card in use.

[0009] This method is described referring to FIG. 18. The noncontact ICcard is set such that the resonant frequency f0 of its resonant circuitis higher than the carrier frequency fc as shown in FIG. 18(A) when thesingle noncontact IC card is used.

[0010] Shown in FIG. 18(B), on the other hand, is a case where twononcontact IC cards overlaid with each other are used. Since the twononcontact IC cards are used together and the two noncontact IC cardsare subject to the influence of mutual inductance, the resonantfrequency f0′ becomes lower than the carrier frequency fc.

[0011] Regardless of whether the single noncontact IC card is used orthe two noncontact IC cards are used together, however, the resonantfrequency f0, f0′ does not deviate so much from the carrier frequency fcas compared to the aforementioned case described with reference to FIG.17. It is therefore possible to decrease the drop of inducedelectromotive force compared to the case described with reference toFIG. 17.

[0012] Another previously proposed method is such that a switch isseries-connected to the resonant circuit and a judgment is made todetermine whether the noncontact IC card is overlaid with anothernoncontact IC card by monitoring the voltage of the resonant circuitwhile turning on and off the switch at random. If it is judged that thenoncontact IC cards are overlaid with each other, the switch of one ofthe noncontact IC cards is turned off and the resonant circuit of theother noncontact IC card brought to a state of resonance. In thismethod, one noncontact IC card is in the state of resonance while theother noncontact IC card is not in the state of resonance when they areoverlaid.

[0013] With this arrangement, it is possible to obtain the samereceiving voltage as would be obtained when one of the noncontact ICcards is used alone. In addition, the other noncontact IC card canreceive energy of the resonant circuit of the first noncontact IC cardthanks to electromagnetic coupling caused by mutual inductance. Thus,either of the two noncontact IC cards can obtain the intended voltage(Japanese Laid-open Patent Publication No. 10-126318).

[0014] Either of the aforementioned methods, however, has drawbacksstated below:

[0015] (1) According to the method of offsetting the resonant frequencyof a noncontact IC card from the carrier frequency, the resonantfrequency is offset from the carrier frequency even when the singlenoncontact IC card is used. Thus, the induced electromotive forcedecreases as much as the amount of frequency offset and thecommunication range decreases.

[0016] (2) According to the method of randomly turning on and off theswitch connected to the resonant circuit, it is necessary to keep areceiver section and a clock circuit inside the noncontact IC cardoperating continuously to detect whether the noncontact IC card isoverlaid with another one. For this reason, power consumption of thenoncontact IC card increases and it requires a large magnetic field. Itis necessary for the noncontact IC card to communicate at a short rangein order that it can receive this large magnetic field.

[0017] It is an object of the invention to provide noncontactcommunications media which can communicate when multiple noncontactcommunications media are overlaid.

[0018] It is another object of the invention to provide noncontactcommunications systems using the aforementioned noncontactcommunications media.

DISCLOSURE OF THE INVENTION

[0019] To solve the foregoing problems, a noncontact communicationsmedium of this invention has the following structure.

[0020] (1) The noncontact communications medium comprises a resonantcircuit including an antenna coil which generates an inducedelectromotive force and thereby transmits and receives signals byreceiving electromagnetic waves carrying information as a result ofchanges in the amount of electric power, and a capacitorparallel-connected to the antenna coil;

[0021] a main circuit which processes a received signal obtained bydemodulating an output of the aforesaid resonant circuit and outputs amodulated transmitting signal to the aforesaid resonant circuit; and

[0022] capacitance change control means which varies the capacitance ofthe aforesaid capacitor according to the amount of an output voltageoccurring in the aforesaid resonant circuit and according to a specificchange in the aforesaid output voltage.

[0023] In the noncontact communications medium of the above structure,when the antenna coil receives electromagnetic waves of a specificfrequency, the resonant circuit resonates and a voltage Q times as highas the induced electromotive force occurs, where Q is the qualityfactor. The resonant circuit is a parallel-resonant circuit providedwith the antenna coil and two capacitors. The two capacitors areconnected parallel to each other.

[0024] The capacitance change control means has a structure for varyingthe capacitance of the capacitor and is realized by a switch section anda switch control section described below.

[0025] The switch section is formed of an analog switch for changing theresonant frequency of the resonant circuit. The analog switch isinserted into the resonant circuit in series with first one of thecapacitors. When the switch section is turned on, the twoparallel-connected capacitors and the antenna coil together constitute aresonant circuit in a connected state in which they are electricallyconnected. On the contrary, when the switch section is turned off, thereis formed a resonant circuit in another connected state in which thefirst capacitor and the antenna coil are electrically disconnected andonly the second capacitor is electrically connected to the antenna coil.The capacitance of the capacitor of the resonant circuit varies due toon/off operation of the switch section.

[0026] The switch control section, which has not been provided inconventional noncontact communications media, controls switchingoperation of the aforesaid switch section according to the amount of anoutput voltage occurring in the aforesaid resonant circuit and accordingto a specific change in the aforesaid output voltage. The aforementionedspecific change means a relatively sharp rising edge of the outputvoltage of the resonant circuit, or a relatively sharp falling edge ofthe output voltage, that accompanies a change in the electromagneticwaves received by the resonant circuit.

[0027] When the single noncontact communications medium is used, theoutput voltage from the resonant circuit increases to or beyond aspecific value if an external communications apparatus with whichcommunication should be carried out and the resonant circuit of thenoncontact communications medium are electromagnetically coupled. If arelatively sharp rising edge or a relatively sharp falling edgeconstituting a specific change in the output voltage occurs when theamount of the output voltage is equal to or larger than the specificvalue, the switch control section compares the output voltage obtainedwhen the first capacitor is set to the connected state by controllingthe switch section with the output voltage obtained when the firstcapacitor is set to a disconnected state, and judges in which state theoutput voltage becomes higher.

[0028] If the capacitances of the two capacitors are determined suchthat the resonant frequency of the resonant circuit and the frequency ofthe electromagnetic waves received by the antenna coil approximatelymatch each other when the aforesaid first capacitor is in the connectedstate, the output voltage of the resonant circuit becomes higher whenthe first capacitor is set to the connected state. The switch controlsection holds the switch section in the state in which the outputvoltage has become higher. Thus, the first capacitor is held in theconnected state. As a consequence, the noncontact communications mediumcan maintain a state of resonance of the resonant circuit and supplysufficient electric power to the main circuit, so that the noncontactcommunications medium can carry out stable noncontact communication withthe communications apparatus.

[0029] On the other hand, if there are two noncontact communicationsmedia, each having a card shape, for example, and the two media overlaidwith each other are brought close to a communications apparatus toachieve electromagnetic coupling, the output voltage of the resonantcircuit of each noncontact communications medium rises to or beyond aspecific value. In each of the two noncontact communications media, if aspecific change in the output voltage occurs when the output voltage isequal to or higher than the specific value, the switch control sectioncompares the output voltage obtained when the first capacitor is set tothe connected state by controlling the switch section with the outputvoltage obtained when the first capacitor is set to the disconnectedstate, and judges in which state the output voltage becomes higher.

[0030] Since the aforementioned specific change accompanies a change inthe electromagnetic waves received by the resonant circuit, it ispossible to synchronize the beginning of controlling the on/offoperation of the switch sections performed by the switch controlsections of the two noncontact communications media.

[0031] There is mutual inductance between the resonant circuits of thetwo noncontact communications media. For this reason, if the capacitanceof the second capacitor is determined such that the resonant frequencyof the resonant circuit and the frequency of the electromagnetic wavesreceived by the antenna coil approximately match each other in eachnoncontact communications medium when the two noncontact communicationsmedia whose first capacitors are set to the disconnected state areoverlaid with each other, the output voltage of the resonant circuitbecomes higher when the first capacitor is set to the disconnected statein each noncontact communications medium. The switch control sectionholds the switch section in the state in which the output voltage hasbecome higher. Thus, the first capacitor is held in the disconnectedstate. As a consequence, the two noncontact communications media canmaintain a state of resonance of the resonant circuits and supplysufficient electric power to the main circuits, so that the twononcontact communications media can carry out stable noncontactcommunication with the communications apparatus.

[0032] The resonant frequency of the resonant circuit becomes lower whenthree noncontact communications media are overlaid than when twononcontact communications media are overlaid. Thus, the capacitance ofthe second capacitor may be determined such that the resonant frequencyof the resonant circuit becomes slightly higher than the frequency ofthe electromagnetic waves received by the antenna coil when twononcontact communications media whose first capacitors are set to thedisconnected state are overlaid with each other. If this arrangement isused, the resonant frequency of the resonant circuit does not greatlydeviate from the frequency of the electromagnetic waves in eachnoncontact communications medium when the aforesaid first capacitor isin the disconnected state, regardless of whether two noncontactcommunications media are overlaid or three noncontact communicationsmedia are overlaid. For this reason, either when two noncontactcommunications media are overlaid, or when three noncontactcommunications media are overlaid, it is possible to avoid a decrease inthe output voltage of the resonant circuit of each noncontactcommunications medium and each noncontact communications medium, so thatthe individual noncontact communications media can carry out stablenoncontact communication with the communications apparatus.

[0033] Again, the capacitances of the two capacitors are to bedetermined such that the resonant frequency of the resonant circuit andthe frequency of the electromagnetic waves received by the antenna coilapproximately match each other in this case when the single noncontactcommunications medium is used with the aforesaid first capacitor in theconnected state. With this arrangement, the communication range of thenoncontact communications medium does not decrease when used alone.

[0034] To increase the resonant frequency of the resonant circuit ofeach noncontact communications medium when two noncontact communicationsmedia whose first capacitors are in the disconnected state are overlaidwith each other, the capacitances of the second capacitors should bemade slightly small. This would be achieved if the capacitances of thefirst capacitor is made slightly larger and the frequency of theaforesaid electromagnetic waves and the resonant frequency of theaforesaid resonant circuit are made approximately equal to each other.

[0035] The noncontact communications media become easier to overlay witheach other and accommodate in a wallet or a pass holder by shaping theminto a card form.

[0036] Also, it is possible to make batteries unnecessary if the outputvoltage of the resonant circuit is used as electric power for thenoncontact communications medium itself.

[0037] Furthermore, since the electric power for operating the maincircuit is supplied after the amount of the output voltage of theresonant circuit has reached a sufficient voltage for operating the maincircuit, it is possible to prevent malfunction of the main circuitpotentially caused by insufficient electric power.

[0038] Furthermore, since the electric power is not supplied to the maincircuit under the control of the switch control section until the amountof the output voltage of the resonant circuit occurring in the resonantcircuit reaches a sufficient voltage for operating the main circuit,power consumption in the noncontact communications medium is low duringthat waiting period. Therefore, even if the noncontact communicationsmedium is present within the communication range, in which the maincircuit can communicate, and the output voltage of the noncontactcommunications medium is low, the switch control section operates,making it possible to determine the output of the switch controlsection. Then, the noncontact communications medium whose switch controlsection has been determined detects that an output voltage sufficientfor operating the main circuit has occurred in the resonant circuit whenthe noncontact communications medium has been brought to thecommunication range at which the main circuit can operate. Since thenoncontact communications medium begins to supply electric power foroperating the main circuit by detecting this output voltage, thenoncontact communications medium immediately begins to operate when itenters the communication range.

[0039] It is preferable that the switch control section be held in thedisconnected state in an initial state. This is because the analogswitch is in its OFF state when the voltage of the electric power isequal to or lower than a specific voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is an external view of a noncontact automatic ticketchecker which uses a noncontact IC card according to an embodiment ofthis invention;

[0041]FIG. 2 is a configuration diagram of a boarding informationread/write device;

[0042]FIG. 3 is a configuration diagram of the noncontact IC card;

[0043]FIG. 4 is an external view of the noncontact IC card;

[0044]FIG. 5 is a configuration diagram of a switch control circuit;

[0045]FIG. 6 is a diagram showing electromagnetic waves output from aticket checker unit and output voltage of a power circuit of thenoncontact IC card;

[0046]FIG. 7 is a diagram explaining ON/OFF operation of a switchelement and a power supply switch element performed until one noncontactIC card becomes ready to operate;

[0047]FIG. 8 is a diagram showing the state of the switch element in asingle noncontact IC card and the output voltage of the power circuit ofthe noncontact IC card;

[0048]FIG. 9 is a diagram explaining ON/OFF operation of the switchelement and the power supply switch element when two noncontact IC cardsoverlaid with each other are used;

[0049]FIG. 10 is a diagram showing the state of the switch elements intwo noncontact IC cards and the output voltage of the power circuits ofthe noncontact IC cards;

[0050]FIG. 11 is a diagram showing transition of the state of thenoncontact IC card associated with a diagram showing ON/OFF states ofthe switch element and the power supply switch element;

[0051]FIG. 12 is a diagram showing variations in the output voltage ofeach card when the single noncontact IC card is used;

[0052]FIG. 13 is a diagram showing variations in the output voltage ofeach card when two noncontact IC cards are used together;

[0053]FIG. 14 is a flowchart showing the operation of the switch controlcircuit;

[0054]FIG. 15 is a diagram showing results of comparison between thenoncontact IC card of the present embodiment and a conventionalnoncontact IC card;

[0055]FIG. 16 is a diagram showing electromagnetic waves output from aticket checker unit according to another embodiment of the invention andoutput voltage of a noncontact IC card;

[0056]FIG. 17 is a diagram for explaining a conventional noncontact ICcard; and

[0057]FIG. 18 is a diagram for explaining a conventional noncontact ICcard.

BEST MODES FOR CARRYING OUT THE INVENTION

[0058]FIG. 1 shows a noncontact IC card which is a noncontactcommunications medium according to an embodiment of the invention and anoncontact automatic ticket checker which performs radio communicationwith the noncontact IC card.

[0059] This automatic ticket checker 1 comprises a pair of ticketchecker units 3 facing each other on both sides of a ticket checkerpassage 2. There are provided doors (not shown) on side surfaces of theindividual ticket checker units 3 for allowing or prohibiting passage ofa user through the ticket checker passage 2.

[0060] The ticket checker units 3 are each provided with an informationread/write device 4. An antenna section 5 constituting part of theinformation read/write device 4 is provided on a top surface of eachticket checker unit 3 so that a wide area of communicability isobtained. When a noncontact IC card 6 used as a ticket is placed withinthe communicability area of the information read/write device 4,noncontact data communication is performed between the noncontact ICcard 6 and the information read/write device 4. More specifically, whena passenger, who is the user carrying the noncontact IC card 6, placesit within the communicability area formed by the antenna section 5 ofthe information read/write device 4, the noncontact IC card 6 performsnoncontact data communication concerning boarding information with theinformation read/write device 4. The automatic ticket checker 1 controlsdoor opening/closing actions in accordance with the results of thiscommunication, thereby allowing or prohibiting passage of the user suchas the passenger.

[0061]FIG. 2 is a configuration diagram of the aforementionedinformation read/write device 4.

[0062] The information read/write device 4 includes the antenna section5, a transmitter section 11, a receiver section 12, a central processingunit (CPU) 14, a read-only memory (ROM) 15, and a random-access memory(RAM) 16.

[0063] The transmitter section 11 generates a carrier for transmittingelectric power to the noncontact IC card 6 and a high-frequency signalby modulating the aforementioned carrier for transmitting data, andsupplies them to the antenna section 5.

[0064] The receiver section 12 demodulates a signal received by theantenna section 5.

[0065] The antenna section 5 is provided with a resonant circuit formedof an antenna coil and capacitors. The antenna section 5 transmits themodulated carrier fed from the transmitter section 11 in the form ofelectromagnetic waves via the resonant circuit. When the noncontact ICcard 6 approaches the antenna section 5, the noncontact IC card 6receives the electromagnetic waves from the antenna section 5 and aresonant circuit provided in the noncontact IC card 6 produces avoltage. The configuration of the noncontact IC card 6 will be laterdescribed. The noncontact IC card 6 utilizes the voltage generated atthis time as a power source. The noncontact IC card 6 also receives asignal transmitted from the antenna section 5, that is, thehigh-frequency signal obtained by modulating the carrier. An inducedelectromotive force generated by the antenna section 5 varies in thereceiver section 12 due to changes in a magnetic field extending fromthe noncontact IC card 6 toward the antenna section 5. This enables thereceiver section 12 to receive the signal transmitted from thenoncontact IC card 6.

[0066] The ROM 15 stores a program necessary for processing operationperformed by the CPU 14 as well as data, such as user identificationinformation and identification number of the noncontact IC card 6.

[0067] The RAM 16 stores data to be transmitted to the automatic ticketchecker 1 as well as data based on the received signal.

[0068] The CPU 14 controls individual structural parts of theinformation read/write device 4 based on the data stored in the ROM 15and the RAM 16.

[0069]FIG. 3 is a configuration diagram of the noncontact IC card.

[0070] This noncontact IC card 6 includes an antenna coil L, twocapacitors C1, C2, a switch element SW, a power circuit 62, a powersupply switch element PSW, a switch control circuit 61, a demodulatingcircuit 64, a modulating circuit 65 and a data processing circuit 63.

[0071] A resonant circuit 60 is formed of the antenna coil L and the twocapacitors C1, C2. The capacitor C1 and the capacitor C2 are connectedparallel to each other. Further, these capacitors C1, C2 areparallel-connected to the antenna coil L. The resonant circuit 60 is aparallel-resonant circuit formed of a coil and capacitors. The switchelement SW is series-connected to the capacitor C2. The capacitor C2 isswitched between a state in which the capacitor C2 is connected to theresonant circuit 60 and a state in which the capacitor C2 isdisconnected from the resonant circuit 60, as the switch element SW isturned on or off. The resonant circuit 60 constitutes an antennasection, and the antenna coil L constitutes an antenna face of theantenna section.

[0072] The antenna coil L is a conductor pattern formed on a surface ofthe noncontact IC card 6 as illustrated in FIG. 4. The center of thenoncontact IC card 6 approximately matches the center of the antennacoil L.

[0073] The capacitances of the two capacitors C1, C2 are determined suchthat the resonant frequency of the resonant circuit 60 when thecapacitor C2 is connected to the antenna coil L (when the switch elementSW is on) approximately matches the frequency of the electromagneticwaves transmitted from the information read/write device 4. Also, thecapacitance of the capacitor C1 is determined such that, when twononcontact IC cards 6 are overlaid with each other, with the capacitorC2 of each noncontact IC card 6 disconnected from the antenna coil L(i.e., the switch element SW is turned off), the resonant frequency ofthe resonant circuit 60 of each noncontact IC card 6 approximatelymatches the frequency of the electromagnetic waves transmitted from theticket checker unit 3 under the influence of mutual inductance betweenthese noncontact IC cards 6.

[0074] The switch control circuit 61 produces a capacitor-connectedstate in which the capacitor C2 is connected to the resonant circuit 60by controlling the switch element SW to turn on. Also, the switchcontrol circuit 61 produces a capacitor-disconnected state in which thecapacitor C2 is disconnected from the resonant circuit 60 by controllingthe switch element SW to turn off. The resonant frequency of theresonant circuit 60 varies as a result of turning on and off of theswitch element SW.

[0075] The capacitor C2 corresponds to what is called a first capacitorin this invention, while the capacitor C1 corresponds to what is calleda second capacitor in this invention.

[0076] The power circuit 62 produces electric power used in the switchcontrol circuit 61, the demodulating circuit 64, the modulating circuit65 and the data processing circuit 63 by rectifying ac power generatedin the resonant circuit 60 into dc power. The power circuit 62 alsodelivers a reset signal (Reset-A) which prohibits operation to theswitch control circuit 61 when output voltage V of the power circuit 62is less than a first threshold Vth1. When this reset signal (Reset-A)has been entered, the switch control circuit 61 does not operate whenthe output voltage V of the power circuit 62 is less than the firstthreshold Vth1.

[0077] The power supply switch element PSW is provided between the powercircuit 62 and the demodulating circuit 64, the modulating circuit 65and the data processing circuit 63. The switch control circuit 61supplies electric power to the demodulating circuit 64, the modulatingcircuit 65 and the data processing circuit 63 by controlling the powersupply switch element PSW to turn on. Also, the switch control circuit61 interrupts power supply to the demodulating circuit 64, themodulating circuit 65 and the data processing circuit 63 by controllingthe power supply switch element PSW to turn off.

[0078] The demodulating circuit 64 demodulates a modulated carrier fedfrom the resonant circuit 60 and outputs a demodulated signal to thedata processing circuit 63.

[0079] The modulating circuit 65 modulates a signal fed from the dataprocessing circuit 63 and outputs a resultant signal to the resonantcircuit 60.

[0080] The data processing circuit 63 stores specific data and performssuch operations as to output a specific signal to the modulating circuit65, store data corresponding to input/output signals, or modify thestored data in accordance with the signal entered from the demodulatingcircuit 64. Here, the demodulating circuit 64, the modulating circuit 65and the data processing circuit 63 are referred to as a data processingsection 66 as a whole. This data processing section 66 corresponds towhat is called a main circuit in this invention.

[0081] The aforementioned switch control circuit 61 monitors the outputvoltage V rectified by the power circuit 62 and controls switchingon/off of the switch element SW and the power supply switch element PSWbased on the value of the output voltage V.

[0082]FIG. 5 is a block diagram showing the functional configuration ofthe switch control circuit 61, in which the aforementioned reset signal(Reset-A) entered from the power circuit 62 to the switch controlcircuit 61 is not shown.

[0083] The switch control circuit 61 includes a first voltage comparator71, a second voltage comparator 72, a voltage change detector 73, aswitch element switcher 74 and a state detector 75. The first voltagecomparator 71 detects whether the output voltage V of the power circuit62 has risen to or beyond the first threshold Vth1 and outputs a signal(Reset-B) indicating the result of detection. The second voltagecomparator 72 detects whether the aforementioned output voltage V hasrisen to or beyond a second threshold Vth2 and outputs a signal (PSWcontrol signal) indicating the result of detection. This PSW controlsignal is a signal which switches the power supply switch element PSWbetween ON and OFF states. The second threshold Vth2 is a voltage higherthan the first threshold Vth1, and is a sufficient voltage for operatingthe data processing section 66. The voltage change detector 73 detectschanges in the aforementioned output voltage V. The switch elementswitcher 74 outputs a signal which turns on and off the switch elementSW. When the switch element SW is turned on and off more than once, thestate detector 75 compares the aforementioned output voltage V duringOFF state and the output voltage V during ON state, judges in whichstate the output voltage V becomes higher, and outputs the result ofdetection. This judgment operation is referred to as detecting operationin this context. As will be described later, the noncontact IC card 6performs this detecting operation when the aforementioned output voltageV is equal to or higher than the first threshold Vth1. Then, thenoncontact IC card 6 maintains the state of the switch element SW (ON orOFF state of the switch element SW) in which the output voltage Vbecomes higher, and waits in that state until the output voltage Vbecomes equal to or higher than the second threshold Vth2.

[0084] Next, the operation of the switch control circuit 61 of thenoncontact IC card 6 of the present embodiment is explained. Thenoncontact IC card 6 of this embodiment is in its initial state when itis kept sufficiently far away from the antenna section 5 of the ticketchecker unit 3 and the output voltage V of the power circuit 62 is lessthan the first threshold Vth1. Reference is made to FIGS. 7, 9 and 11with respect to this initial state and later-described states 1 to 6. Inthe initial state, both the switch element SW and the power supplyswitch element PSW are in their OFF state.

[0085] When the output voltage V of the power circuit 62 is less thanthe first threshold Vth1, the switch control circuit 61 does not operatebecause the reset signal (Reset-A) which prohibits the operation isentered from the power circuit 62. Thus, when the aforementioned outputvoltage V is less than the first threshold Vth1, the first voltagecomparator 71 would in no case erroneously detect that theaforementioned output voltage V is equal to or higher than the firstthreshold Vth1 thereby causing the switch control circuit 61 toerroneously start the operation described below.

[0086] The output voltage V of the aforementioned power circuit 62 risesas the noncontact IC card 6 approaches the antenna section 5 of theticket checker unit 3. When the output voltage V becomes equal to orhigher than the first threshold Vth1, the power circuit 62 stops toenter the aforementioned reset signal (Reset-A) into the switch controlcircuit 61. As a result, the switch control circuit 61 of the noncontactIC card 6 starts to operate and the first voltage comparator 71 detectsthat the aforementioned output voltage V is equal to or higher than thefirst threshold Vth1. This condition is referred to as state 1 for theconvenience of explanation. The state 1 is the condition in which boththe switch element SW and the power supply switch element PSW are intheir OFF state.

[0087] The ticket checker unit 3 transmits a polling command to anynoncontact IC card 6 which is placed within the communicability area atspecific time intervals (e.g., every 10 ms) requesting the noncontact ICcard 6 to return a response. Since the electromagnetic wave transmittedfrom the ticket checker unit 3 is modulated as shown in FIG. 6(A), theoutput voltage V of the power circuit 62 slightly drops as shown in FIG.6(B) at this time. If the detecting operation is performed to determinethe state of the switch element SW in which the output voltage V becomeshigher by turning on and off the switch element SW more than once whilethis polling command is being received (during the reception of themodulated wave), it is not possible to judge whether the change in theoutput voltage V is a voltage change due to ON/OFF operation of theswitch element SW or a voltage change caused by the modulated wave. Forthis reason, it is impossible to correctly determine the state of theswitch element SW in which the aforementioned output voltage V becomeshigher.

[0088] When a plurality of noncontact IC cards 6 are overlaid and theindividual noncontact IC cards 6 start the detecting operation at anygiven point in time, the aforementioned output voltage V varies due to achange in the state (ON/OFF state of the switch element SW) of anothernoncontact IC card 6. For this reason, it is impossible to correctlydetermine the state of the switch element SW in which the aforementionedoutput voltage V becomes higher.

[0089] Under this circumstance, the noncontact IC card 6 of the presentembodiment carries out the operation described below to correctlydetermine the state of the switch element SW in which the aforementionedoutput voltage V becomes higher.

[0090] When the ticket checker unit 3 is transmitting a polling command,the output voltage V of the power circuit 62 is at a low level becausethe electromagnetic wave transmitted from the ticket checker unit 3 ismodulated. It is possible to detect the end of transmission of theaforementioned polling command by detecting a rising edge of the outputvoltage V.

[0091] The voltage change detector 73 serves to detect this rising edgeof the output voltage V. The noncontact IC card 6 initiates theaforementioned detecting operation by reference to the detected risingedge of the output voltage V. This makes it possible to perform thedetecting operation in a period other than when the electromagnetic wavetransmitted from the ticket checker unit 3 is modulated and tosynchronize execution of the detecting operation in each of the multiplenoncontact IC cards 6 when they are overlaid. As a consequence, thenoncontact IC card 6 can correctly determine the state of the switchelement SW in which the aforementioned output voltage V becomes higher.

[0092] It is to be pointed out that the aforementioned detectingoperation can be performed within a shorter period (e.g., 4 to 5 ms)than the polling command transmitting time interval (10 ms).

[0093] The rising edge of the output voltage V of the power circuit 62at the end of transmission of the aforementioned polling command fromthe ticket checker unit 3 is relatively sharp. On the other hand, risingof the output voltage V of the power circuit 62 when the noncontact ICcard 6 is brought close to the antenna section 5 of the ticket checkerunit 3 is relatively slow. The voltage change detector 73 detects therelatively sharp rising edge of the aforementioned output voltage V asthe end of transmission of the aforementioned polling command from theticket checker unit 3, but the voltage change detector 73 does notdetect the relatively slow rising of the output voltage V as the end oftransmission of the aforementioned polling command.

[0094] In the noncontact IC card 6, the voltage change detector 73detects the sharp rising edge of the output voltage V when the outputvoltage v of the aforementioned power circuit 62 is equal to or higherthan the first threshold Vth1. The voltage change detector 73 detectsthis rising edge of the output voltage V. This condition is referred toas state 2 for the convenience of explanation. The state 2 is thecondition in which the detecting operation is executed by turning on andoff the switch element SW more than once at the fixed time intervals.The state 2 is also the condition in which the power supply switchelement PSW is fixed to its OFF state.

[0095] The noncontact IC card 6 causes the switch element SW to turn onand off more than once at the fixed time intervals in the detectingoperation. This is intended to enable a plurality of noncontact IC cards6 become on and off almost simultaneously. If a judgment is made thatthe output voltage V of the power circuit 62 becomes higher when theswitch element SW is on than when the switch element SW is off, theswitch element SW is maintained in its ON state. This condition isreferred to as state 3 for the convenience of explanation. The state 3is the condition in which the switch element SW is on and the powersupply switch element PSW is off. Contrary to this, if a judgment ismade that the output voltage V of the power circuit 62 becomes higherwhen the switch element SW is off than when the switch element SW is on,the noncontact IC card 6 holds the switch element SW in its OFF state.This condition is referred to as state 4 for the convenience ofexplanation. The state 4 is the condition in which both the switchelement SW and the power supply switch element PSW are in their OFFstate. The capacitor C2 is electrically connected to the resonantcircuit 60 when the switch element SW is on, and the capacitor C2 iselectrically disconnected from the resonant circuit 60 when the switchelement SW is off.

[0096] The switch control circuit 61 further controls the power supplyswitch element PSW in the following fashion.

[0097] When the second voltage comparator 72 detects that theaforementioned output voltage V has risen beyond the second thresholdVth2 (Vth2>Vth1) in the state 3 or 4, the noncontact IC card 6 causesthe power supply switch element PSW to turn on. A condition in which thepower supply switch element PSW has become ON from the state 3 isreferred to as state 5 for the convenience of explanation. Also, acondition in which the power supply switch element PSW has become ONfrom the state 4 is referred to as state 6 for the convenience ofexplanation. The state 5 is the condition in which both the switchelement SW and the power supply switch element PSW are in their ONstate, while the state 6 is the condition in which switch element SW isin its OFF state and the power supply switch element PSW is in its ONstate.

[0098] Whether the noncontact IC card 6 takes the state 5 or the state 6depends on whether the state preceding the state 5 or the state 6 hasbeen the state 3 or the state 4. The state 5 is a condition in which thetwo capacitors C1, C2 connected parallel to the antenna coil L areconnected. The state 6 is a condition in which the capacitor C1 isconnected but the capacitor C2 is not connected to the antenna coil L.While the resonant frequency of the resonant circuit 60 differs betweenthe state 5 or the state 6, the output voltage V of the power circuit 62rises beyond the specific threshold Vth2 and a voltage sufficient foroperating the data processing section 66 is obtained in either of thesestates. Described below in detail are reasons why this situation isrealized.

[0099]FIG. 7 is a diagram showing results of control operation performedby the switch control circuit 61 over the switch element SW and thepower supply switch element PSW when the noncontact IC card 6 is broughtclose to the antenna section 5 of the ticket checker unit 3.

[0100] In the initial state, the switch element SW is in its OFF state,and the power supply switch element PSW is also in its OFF state. Whenthe noncontact IC card 6 approaches the antenna section 5 in thisinitial state, an induced electromotive force occurs in the resonantcircuit 60 of the noncontact IC card 6 and the output voltage V of thepower circuit 62 rises. Then, when the aforementioned output voltage Vbecomes equal to or higher than the first threshold Vth1, the powercircuit 62 stops to enter the reset signal (Reset-A) to the switchcontrol circuit 61, causing the switch control circuit 61 to initiateits operation. When the first voltage comparator 71 of the switchcontrol circuit 61 detects that the output voltage V has become equal toor higher than the specified threshold Vth1, the noncontact IC card 6goes into the state 1.

[0101] When the voltage change detector 73 detects a relatively sharprising edge of the output voltage V of the power circuit 62, or thevoltage change detector 73 detects the end of transmission of thepolling command from the ticket checker unit 3, while the noncontact ICcard 6 is in the state 1, the noncontact IC card 6 goes into the state2. In this state 2, the noncontact IC card 6 executes the detectingoperation to determine the state of the switch element SW in which theoutput voltage V of the power circuit 62 becomes higher by turning onand off the switch element SW more than once.

[0102] As previously stated, the capacitances of the two capacitors C1,C2 are determined such that the resonant frequency of the resonantcircuit 60 when the capacitor C2 is connected to the antenna coil Lapproximately matches the frequency of the electromagnetic wavestransmitted from the ticket checker unit 3.

[0103] When the single noncontact IC card 6 is used, the output voltageV of the power circuit 62 becomes higher when the switch element SW isturned on and the capacitor C2 is connected to the antenna coil L in theabove-described detecting operation (see FIG. 8). FIG. 8(A) shows ON/OFFstates of the switch element SW and FIG. 8(B) shows the output voltage Vof the power circuit 62. As a result, the noncontact IC card 6 goes intothe state 3 in which the switch control circuit 61 holds the switchelement SW in the ON state. At this point, the resonant frequency of theresonant circuit 60 approximately matches the frequency of theelectromagnetic waves transmitted from the ticket checker unit 3.

[0104] Subsequently, the second voltage comparator 72 detects that theoutput voltage V of the power circuit 62 has risen to or beyond thesecond threshold Vth2, and the noncontact IC card 6 goes into the state5 in which the power supply switch element PSW is in the ON state.

[0105] When the noncontact IC card 6 is brought close to the antennasection 5 of the ticket checker unit 3, the noncontact IC card 6 shiftsfrom its initial state to the state 1, state 2, state 3 and state 5 inthis order as seen above and becomes ready to perform noncontact datacommunication.

[0106]FIG. 9 is a diagram showing results of control operation performedby the switch control circuit 61 over the switch element SW and thepower supply switch element PSW when two noncontact IC cards 6 overlaidwith each other are brought close to the antenna section 5 of the ticketchecker unit 3. The two noncontact IC cards 6 perform generally the sameoperation. The following explanation deals with the operation of one ofthese noncontact IC cards 6.

[0107] In the initial state, the switch element SW and the power supplyswitch element PSW are in their OFF state. When the stacked twononcontact IC cards 6 in the initial state are brought close to theantenna section 5, an induced electromotive force occurs in the resonantcircuit 60 of each noncontact IC card 6 and the output voltage V of thepower circuit 62 rises. Then, when the aforementioned output voltage Vbecomes equal to or higher than the first threshold Vth1, the powercircuit 62 stops to enter the reset signal (Reset-A) to the switchcontrol circuit 61, causing the switch control circuit 61 to initiateits operation. When the first voltage comparator 71 of the switchcontrol circuit 61 detects that the output voltage V has become equal toor higher than the specified threshold Vth1, the noncontact IC card 6goes into the state 1.

[0108] The two noncontact IC cards 6 go into the state 2 as each of thevoltage change detectors 73 detects a rising edge of the output voltageV of the power circuit 62 at the same time. Then, each of the noncontactIC cards 6 executes the detecting operation to determine the state ofthe switch element SW in which the output voltage V of the power circuit62 becomes higher by turning on and off the switch element SW. Thus, thedetecting operation performed by the two noncontact IC cards 6 issynchronized.

[0109] In the two noncontact IC cards 6, the output voltage V rises toor beyond the first threshold Vth1 due to the induced electromotiveforce occurring in the respective resonant circuits 60 almostsimultaneously, or the output voltage V in one noncontact IC card 6first rises to or beyond the first threshold Vth1 and the output voltageV in the other noncontact IC card 6 rises to or beyond the firstthreshold Vth1.

[0110] In either case, the output voltage V of the power circuit 62rises to or beyond the first threshold Vth1 in both of the twononcontact IC cards 6, and the voltage change detector 73 detects therising edge of the output voltage V of the power circuit 62. The voltagechange detectors 73 of the two noncontact IC cards 6 simultaneouslydetect the rising edge of the output voltage V of the respective powercircuits 62. Consequently, the two noncontact IC cards 6 simultaneouslygo into the state 2 and initiate the detecting operation. The twononcontact IC cards 6 initiate the detecting operation by reference tothe end of transmission of the polling command from the ticket checkerunit 3 in this fashion. Therefore, it is possible to perform thedetecting operation in a period other than when the electromagnetic wavereceived by the resonant circuit 60 is modulated and to synchronize thedetecting operation executed by the two noncontact IC cards 6. Since thetwo noncontact IC cards 6 are preset to become on and off at fixed timeintervals, they turn on and off the respective switch elements SW at thesame timing.

[0111] As previously stated, the capacitance of the capacitor C1 isdetermined such that, when the two noncontact IC cards 6 in which thecapacitor C2 is connected to the antenna coil L are overlaid with eachother, the resonant frequency of the resonant circuit 60 of eachnoncontact IC card 6 approximately matches the frequency of theelectromagnetic waves transmitted from the ticket checker unit 3 underthe influence of mutual inductance between these noncontact IC cards 6.

[0112] When two noncontact IC cards 6 are overlaid with each other, theoutput voltage V of the power circuit 62 becomes higher when the switchelement SW is turned off and the capacitor C2 is disconnected from theantenna coil L (see FIG. 10). FIG. 10(A) shows ON/OFF states of theswitch element SW and FIG. 10(B) shows the output voltage V of the powercircuit 62. As a result, the two noncontact IC cards 6 together go intothe state 4 in which the switch control circuit 61 holds the switchelement SW in the OFF state.

[0113] At this point, although the two noncontact IC cards 6 are underthe influence of the mutual inductance, the resonant frequency of theresonant circuit 60 and the frequency of the electromagnetic wavestransmitted from the ticket checker unit 3 approximately match.

[0114] Subsequently, the second voltage comparator 72 detects that theoutput voltage V of the power circuit 62 has risen to or beyond thesecond threshold Vth2, and the two noncontact IC cards 6 go into thestate 6 in which the power supply switch element PSW is in the ON state.

[0115] Due to the above-described operation of the switch controlcircuit 61, the two noncontact IC cards 6 can supply sufficient electricpower to their data processing sections 66 even when the two noncontactIC cards 6 overlaid with each other are brought close to the antennasection 5 of the ticket checker unit 3. Therefore, both of thenoncontact IC cards 6 become ready to perform noncontact datacommunication.

[0116] Even when three or more noncontact IC cards 6 are overlaid, eachnoncontact IC card 6 follows one of the aforementioned state transitionflows. More specifically, when three or more overlaid noncontact ICcards 6 are used, each noncontact IC card 6 performs the detectingoperation to determine the state (ON or OFF) of the switch element SW inwhich the output voltage of the power circuit 62 becomes higher, andholds the switch element SW in that state.

[0117] Next, transition of the state of the noncontact IC card 6 isdescribed in detail referring to FIG. 11. FIG. 11(A) shows the statetransition flow of the noncontact IC card 6 and FIG. 11(B) shows ON/OFFstates of the switch element SW and the power supply switch element PSWin each state. In the initial state, both the switch element SW and thepower supply switch element PSW are in their OFF state. When thenoncontact IC card 6 is brought close to the antenna section 5 of theticket checker unit 3, the output voltage V of the power circuit 62rises. When the output voltage V rises to or beyond the first thresholdVth1, the noncontact IC card 6 goes into the state 1. Both the switchelement SW and the power supply switch element PSW are in their OFFstate in this state 1 as well.

[0118] When the noncontact IC card 6 detects the end of transmission ofthe polling command from the ticket checker unit 3 in this state 1, thenoncontact IC card 6 goes into the state 2 and initiates the detectingoperation. In this detecting operation, the noncontact IC card 6determines the state of the switch element SW in which the outputvoltage V of the power circuit 62 becomes higher by turning on and offthe switch element SW more than once. More specifically, the noncontactIC card 6 determines the state of the switch element SW in which thedifference between the resonant frequency of the resonant circuit 60 andthe frequency of the electromagnetic waves transmitted from the ticketchecker unit 3 becomes small.

[0119] If a judgment is made that the output voltage becomes higher whenthe switch element SW is on than when the switch element SW is off inthe aforementioned detecting operation, the noncontact IC card 6 goesinto the state 3 in which the switch element SW is held in the ON state.Subsequently, the noncontact IC card 6 waits until the output voltage Vrises to or beyond the second threshold Vth2 and goes into the state 5.In this state 5, the switch control circuit 61 holds the switch elementSW and the power supply switch element PSW in their ON state. At thispoint, the noncontact IC card 6 supplies sufficient electric power toits data processing section 66 and can perform noncontact communicationwith the ticket checker unit 3.

[0120] Upon completion of noncontact communication with the ticketchecker unit 3 in the state 5, the noncontact IC card 6 returns to theinitial state. The noncontact IC card 6 also returns to the initialstate when it is separated from the antenna section 5 and the outputvoltage V of the power circuit 62 drops to less than the first thresholdVth1.

[0121] Conversely, if a judgment is made that the aforementioned outputvoltage V becomes higher when the switch element SW is off in theforegoing detecting operation, the noncontact IC card 6 goes into thestate 4 in which the switch element SW is held in the OFF state.Subsequently, the noncontact IC card 6 waits until the output voltage Vrises to or beyond the second threshold Vth2 and goes into the state 6.In this state 6, the switch control circuit 61 holds the switch elementSW in the OFF state and the power supply switch element PSW in the ONstate. At this point, the noncontact IC card 6 supplies sufficientelectric power to its data processing section 66 and can performnoncontact communication with the ticket checker unit 3.

[0122] Upon completion of noncontact communication with the ticketchecker unit 3 in the state 6, the noncontact IC card 6 returns to theinitial state. The noncontact IC card 6 also returns to the initialstate when it is separated from the antenna section 5 and the outputvoltage V of the power circuit 62 drops to less than the first thresholdVth1.

[0123] The noncontact IC card 6 also returns to the initial state if theoutput voltage V of the power circuit 62 drops to less than the firstthreshold Vth1 when the noncontact IC card 6 is in the state 3 or state4. When the noncontact IC card 6 returns from the state 3 to the initialstate, the switch element SW is turned off.

[0124] While the aforementioned power supply switch element PSW need notnecessarily be provided in the foregoing configuration, the electricpower is always supplied from the power circuit 62 to the dataprocessing section 66 if the power supply switch element PSW is notprovided.

[0125] In contrast, if the power supply switch element PSW is provided,it is possible to interrupt the power supply to the data processingsection 66 by turning off the power supply switch element PSW andthereby reduce power consumption of the noncontact IC card 6. Normally,the data processing section 66 consumes greater electric power than theswitch control circuit 61. Therefore, by interrupting the power supplyto the data processing section 66, it is possible to operate the switchcontrol circuit 61 with weaker electromagnetic waves than when the powersupply to the data processing section 66 is not interrupted.Furthermore, by turning on the power supply switch element PSW after theoutput voltage V of the power circuit 62 has risen to or beyond thesecond threshold Vth2 at which the data processing section 66 can beoperated in a stable fashion, it is possible to prevent malfunction ofthe data processing section 66 due to insufficient power supply.

[0126] In addition, it is possible to determine the state of the switchelement SW controlled by the switch control circuit 61 before thenoncontact IC card 6 is brought to a distance at which sufficient powersupply for operating its main circuit can be supplied. With thisarrangement, electric power is supplied to the main circuit, causing itto operate, when the noncontact IC card 6 has been brought to thatdistance.

[0127] Referring next to FIGS. 12 and 13, time-sequential transition ofthe output voltage V of the power circuit 62 that occurs when thenoncontact IC card 6 is brought close to the antenna section 5 of theinformation read/write device 4 is described in detail.

[0128]FIG. 12 is a diagram showing time-sequential transition of theoutput voltage V of the power circuit 62 that occurs in one noncontactIC card 6 when the noncontact IC card 6 is brought close to the antennasection 5 of the information read/write device 4. Should the noncontactIC card 6 detect the end of transmission of a polling command (at timet5 shown in the Figure) when the output voltage V of the power circuit62 is equal to or higher than the first threshold Vth1 (at time t3 orlater shown in the Figure), the noncontact IC card 6 initiates thedetecting operation. The period between times t1 and t2 and the periodbetween times t4 and t5 are polling command transmitting periods. Basedon results of the detecting operation thus initiated, the switch elementSW is held in the ON state at time t6. Since the resonant frequency ofthe resonant circuit 60 approximately matches the frequency of theelectromagnetic wave transmitted from the ticket checker unit 3 as aconsequence, the output voltage V of the power circuit 62 sharply rises.The output voltage V then continues to rise further. When theaforementioned output voltage reaches the second threshold Vth2 (timet7), the noncontact IC card 6 turns on the power supply switch elementPSW. As a result, the noncontact IC card 6 begins to supply electricpower to its main circuit and power consumption of the noncontact ICcard 6 increases, so that the output voltage V rapidly drops down tovoltage V3 (time t8). Then, the output voltage V rises up to asaturation voltage V4 (time t9) and, subsequently, the output voltagebecomes constant. A reason why the output voltage V sharply rises attime t6 is that the output voltage V instantly rises up to a voltagewhich should have been reached along a voltage-rising curve Q if theresonant circuit had been in a state of resonance from the beginning.Further, the output voltage rapidly drops down to the voltage V3immediately after the power supply switch element PSW has becomes on.The second threshold Vth2 is set such that this output voltage V3 wouldbecome equal to or higher than a voltage necessary for operating thedata processing section 66 which is the main circuit of the noncontactIC card 6.

[0129]FIG. 13 is a diagram showing time-sequential transition of theoutput voltage V of the power circuit 62 that occurs in each noncontactIC card 6 when two noncontact IC cards 6 are brought close to theantenna section 5 of the information read/write device 4. Should any oneof the noncontact IC cards 6 detect the end of transmission of a pollingcommand (at time t5 shown in the Figure) when the output voltage V isequal to or higher than the first threshold Vth1 (at time t3 or latershown in the Figure), the noncontact IC card 6 initiates the detectingoperation. Based on results of this detecting operation, the switchelement SW is held in the OFF state at time t6. As a consequence, theresonant frequency of the resonant circuit 60 which is affected bymutual inductance approximately matches the frequency of theelectromagnetic wave transmitted from the ticket checker unit 3 in eachnoncontact IC card 6. Then, when the aforementioned output voltagereaches the second threshold Vth2 (time t7), the noncontact IC card 6turns on the power supply switch element PSW. As a result, thenoncontact IC card 6 begins to supply electric power to its main circuitand power consumption of the noncontact IC card 6 increases, so that theoutput voltage V rapidly drops down to voltage V3 (time t8). Then, theoutput voltage V rises up to a saturation voltage V4 (time t9) and,subsequently, the output voltage becomes constant. A reason why theoutput voltage V of the power circuit 62 does not sharply rise at timet6 is that the output voltage V has risen along a voltage-rising curve Qsince the switch element SW is in the OFF state in the initial state ofthe noncontact IC card 6.

[0130]FIG. 14 is a flowchart showing the operation of the switch controlcircuit 61.

[0131] At s1, a judgment is made to determine whether the output voltageV of the power circuit 62 is equal to or higher than the first thresholdVth1. If the output voltage V is equal to or higher than the firstthreshold Vth1, the end of transmission of a polling command is detectedat s2. When the end of transmission of the polling command has beendetected (s3), the switch element SW is turned on and off more thanonce, and a judgment is made to determine the state of the switchelement SW in which the output voltage V of the power circuit 62 becomeshigher (s4). If the output voltage V of the power circuit 62 is higherwhen the switch element SW is on, the switch element SW is held in theOFF state (s5), and the noncontact IC card 6 waits until the outputvoltage V becomes equal to or higher than the first threshold Vth1 (s6)or equal to or higher than the second threshold Vth2 (s7). When theoutput voltage V becomes lower than the first threshold Vth1, the switchelement SW is returned to OFF at s8 (s8) and the current operation isfinished. As a result, the noncontact IC card 6 returns to the initialstate.

[0132] When the output voltage V becomes equal to or higher than thesecond threshold Vth2, the power supply switch element PSW is turned on(s9).

[0133] If the output voltage V of the power circuit 62 is higher whenthe switch element SW is turned off at s4, the switch element SW is heldin the OFF state (s10), and the noncontact IC card 6 waits until theoutput voltage V becomes lower than the first threshold Vth1 (s11) orequal to or higher than the second threshold Vth2 (s12). When the outputvoltage V becomes lower than the first threshold Vth1, the currentoperation is finished. As a result, the noncontact IC card 6 returns tothe initial state.

[0134] When the output voltage V becomes equal to or higher than thesecond threshold Vth2, the power supply switch element PSW is turned on(s9).

[0135] The noncontact IC card 6 returns to the initial state whennoncontact communication with the ticket checker unit 3 is completedafter the power supply switch element PSW has been turned on at s9. Thenoncontact IC card 6 also returns to the initial state when it isseparated from the antenna section 5 and the output voltage V of thepower circuit 62 drops to less than the first threshold Vth1.

[0136] In the above-described embodiment, the capacitance of thecapacitor C1 is determined such that, when two noncontact IC cards 6 areoverlaid with each other, with the capacitor C2 of each noncontact ICcard 6 disconnected from the antenna coil L, the resonant frequency ofthe resonant circuit 60 of each noncontact IC card 6 approximatelymatches the frequency of the electromagnetic waves transmitted from theticket checker unit 3 under the influence of mutual inductance betweenthese noncontact IC cards 6. This may be modified as described below.

[0137] Specifically, the capacitance of the capacitor C1 may be suchthat, when two noncontact IC cards 6 are overlaid with each other, withthe capacitor C2 of each noncontact IC card 6 disconnected from theantenna coil L, the resonant frequency of the resonant circuit 60 ofeach noncontact IC card 6 is slightly higher than the frequency of theelectromagnetic waves transmitted from the ticket checker unit 3 underthe influence of mutual inductance between these noncontact IC cards 6.

[0138] It is to be pointed out, however, that the capacitances of thetwo capacitors C1, C2 are determined such that, when the singlenoncontact IC card 6 is used, the resonant frequency of the resonantcircuit 60 when the capacitor C2 is connected to the antenna coil Lapproximately matches the frequency of the electromagnetic wavestransmitted from the ticket checker unit 3.

[0139] The resonant frequency of the resonant circuit 60 becomes lowerwhen three noncontact IC cards 6 are overlaid than when two noncontactIC cards 6 are overlaid. If the capacitances of the capacitors C1, C2are determined as stated above and two noncontact IC cards 6 areoverlaid, or three noncontact IC cards 6 are overlaid, the resonantfrequency of the resonant circuit 60 of each noncontact IC card 6 doesnot greatly deviate from the frequency of the electromagnetic wavestransmitted from the ticket checker unit 3 when the earlier-mentionedfirst capacitor C2 is in the disconnected state. For this reason, eitherwhen two noncontact IC cards 6 are overlaid, or when three noncontact ICcards 6 are overlaid, it is possible to avoid a decrease in the outputvoltage of the resonant circuit 60 of each noncontact IC card 6.Therefore, the individual noncontact IC cards 6 can carry out stablenoncontact communication with the ticket checker unit 3.

[0140] Since the capacitances of the two capacitors C1, C2 aredetermined such that, when the single noncontact IC card 6 is used, theresonant frequency of the resonant circuit 60 when the capacitor C2 isconnected to the antenna coil L approximately matches the frequency ofthe electromagnetic waves transmitted from the ticket checker unit 3,the communication range does not decrease when the single noncontact ICcard 6 is used.

[0141] Results of comparison between the noncontact IC card 6 of thepresent embodiment and a conventional noncontact IC card are shown inFIG. 15. Indicated by “A” is the result obtained when the resonantfrequency is approximately matched to the frequency of theelectromagnetic waves transmitted from the ticket checker unit 3 whentwo noncontact IC cards 6 whose switch elements SW are in the OFF stateare overlaid with each other, and indicated by “B” is the resultobtained when the resonant frequency is slightly increased. Further,indicated by “C” is the result obtained with the conventional noncontactIC card. As shown in FIG. 15, when a plurality of conventionalnoncontact IC cards are overlaid, the resonant frequency greatlydecreases and, therefore, the noncontact IC cards can not communicatewith the ticket checker unit 3 in a stable fashion when they areoverlaid with one another. Contrary to this, the noncontact IC cards 6of this embodiment make it possible to avoid a drop of the resonantfrequency by controlling ON/OFF actions of the switch element SW.Therefore, it is possible to perform noncontact communication with theticket checker unit 3 even in a state where a plurality of noncontact ICcards 6 are overlaid with one another.

[0142] Described next is a method by which the noncontact IC card 6 andthe information read/write device 4 of the ticket checker unit 3 performcommunication.

[0143] First, a method by which the information read/write device 4 ofthe ticket checker unit 3 communicates with one noncontact IC card 6after another existing in the communicability area is explained below.Here, the method is described using an example in which two noncontactIC cards 6 are present within the communicability area.

[0144] The information read/write device 4 transmits a polling commandat specific time intervals until a response to the polling command isreceived.

[0145] When any noncontact IC card 6 existing within the communicabilityarea receives this polling command, the noncontact IC card 6 transmits aresponse containing its own identification information back to theinformation read/write device 4.

[0146] If a plurality of noncontact IC cards 6 exist within the samecommunicability area, such as when two or more noncontact IC cards 6 areoverlaid, these noncontact IC cards 6 synchronize in operation.Therefore, responses to the polling command returned to the informationread/write device 4 interfere with one another. In this circumstance, ananticollision treatment as described below, for example, is performed.

[0147] First, if the information read/write device 4 judges that theresponses returned from the multiple noncontact IC cards 6 after pollingare interfering with one another (data can not be correctly received),the information read/write device 4 reexecutes polling operation. Thepolling command transmitted in this reexecuted polling operation is adifferent command from the polling command transmitted in the firstpolling operation. Here, for the convenience of explanation, the firstpolling command is referred to as the polling command A and the secondpolling command is referred to as the polling command B.

[0148] The multiple noncontact IC cards 6 which have received thepolling command B individually return their responses to the informationread/write device 4 with random time delays. This makes it possible toavoid interference of the responses from the individual noncontact ICcards 6.

[0149] If interference occurs even when the responses are returned withrandom time delays, the information read/write device 4 retransmits thepolling command B, causing the noncontact IC cards 6 to randomly respondagain. The information read/write device 4 repeatedly transmits thepolling command B until the responses from the noncontact IC cards 6 donot interfere with one another. With this arrangement, the informationread/write device 4 can receive interference-free responses from theindividual noncontact IC cards 6.

[0150] Let us now consider a case where one of two noncontact IC cards 6transmits a response at an earlier time.

[0151] The information read/write device 4 receives the response fromone noncontact IC card 6 (hereinafter referred to as the firstnoncontact IC card 6) at first. The information read/write device 4 thendisregards the response transmitted subsequently from the othernoncontact IC card 6 (hereinafter referred to as the second noncontactIC card 6). The information read/write device 4 determines from whichnoncontact IC card 6 the response has been received based on theidentification information of the received response. To begin noncontactcommunication with the first noncontact IC card 6 from which theresponse has been received, the information read/write device 4transmits a command for establishing communication to the firstnoncontact IC card 6. This communication-establishing command containsidentification information of the first noncontact IC card 6.

[0152] Upon receiving this communication-establishing command, the firstnoncontact IC card 6 transmits a response to thecommunication-establishing command back to the information read/writedevice 4.

[0153] When the response to the communication-establishing command isreceived, the information read/write device 4 transmits a read command,etc. to the first noncontact IC card 6. The first noncontact IC card 6then transmits a response corresponding to the received command back tothe information read/write device 4. Noncontact communication betweenthe information read/write device 4 and the first noncontact IC card 6is carried out in this manner.

[0154] When the noncontact communication with the first noncontact ICcard 6 is finished, the information read/write device 4 retransmits thepolling command. Since the noncontact communication is finished at thispoint, the first noncontact IC card 6 does not transmit any response tothis polling command. The second noncontact IC card 6, which has notfinished the noncontact communication transmits the response to thispolling command.

[0155] Upon receiving this response, the information read/write device 4transmits a communication-establishing command to the second noncontactIC card 6.

[0156] When the communication-establishing command is received, thesecond noncontact IC card 6 transmits a response to thiscommunication-establishing command back to the information read/writedevice 4.

[0157] When the response to this communication-establishing command isreceived, the information read/write device 4 transmits a read command,etc. to the second noncontact IC card 6. The second noncontact IC card 6transmits a response corresponding to the received command back to theinformation read/write device 4. Noncontact communication between theinformation read/write device 4 and the second noncontact IC card 6 iscarried out in this way.

[0158] The information read/write device 4 transmits the polling commandat specific times and performs noncontact communication with thenoncontact IC card 6 which has returned the response to thecommunication-establishing command in the aforementioned manner.

[0159] Described next is another method by which the informationread/write device 4 communicates with a plurality of noncontact IC cards6 existing in the communicability area. Here again, the method isdescribed using an example in which two noncontact IC cards 6 arepresent within the communicability area.

[0160] The information read/write device 4 transmits the polling commandat specific time intervals until the response to the polling command isreceived.

[0161] When any noncontact IC card 6 existing within the communicabilityarea receives this polling command, the noncontact IC card 6 transmits aresponse containing its own identification information back to theinformation read/write device 4. If interference occurs here, theanticollision treatment is performed. Let us now consider a case whereone of two noncontact IC cards 6 transmits a response at an earliertime.

[0162] The information read/write device 4 receives the response fromone noncontact IC card 6 (hereinafter referred to as the firstnoncontact IC card 6) at first. Subsequently, the information read/writedevice 4 receives the response from the other noncontact IC card 6(hereinafter referred to as the second noncontact IC card 6).

[0163] The information read/write device 4 which has received theresponses from the two noncontact IC cards 6 retransmits the pollingcommand to judge whether there exists any other noncontact IC card 6within the communicability area with which communication can be made. Ifthe information read/write device 4 judges that no response is receivedfrom other noncontact IC card 6, the information read/write device 4performs an operation for carrying out noncontact communication with thetwo noncontact IC cards 6.

[0164] The information read/write device 4 determines from whichnoncontact IC card 6 each response has been received based on theidentification information of the received response. The informationread/write device 4 transmits a command for establishing communicationto each noncontact IC card 6. Upon receiving thiscommunication-establishing command, the first noncontact IC card 6transmits a response to the communication-establishing command back tothe information read/write device 4. Upon receiving thecommunication-establishing command, the second noncontact IC card 6 alsotransmits a response to the communication-establishing command back tothe information read/write device 4.

[0165] Upon receiving the responses to the communication-establishingcommands from the individual noncontact IC cards 6, the informationread/write device 4 transmits a read command, etc. to the firstnoncontact IC card 6. The first noncontact IC card 6 then transmits aresponse corresponding to the received command back to the informationread/write device 4. Noncontact communication between the informationread/write device 4 and the first noncontact IC card 6 is carried out inthis manner.

[0166] When the noncontact communication with the first noncontact ICcard 6 is finished, the information read/write device 4 transmits a readcommand, etc. to the second noncontact IC card 6. The second noncontactIC card 6 transmits a response corresponding to the received commandback to the information read/write device 4. Noncontact communicationbetween the information read/write device 4 and the second noncontact ICcard 6 is carried out in this way.

[0167] When the noncontact communication with the second noncontact ICcard 6 is finished, the information read/write device 4 transmits thepolling command.

[0168] The information read/write device 4 repeatedly transmits thepolling command at specific time intervals until the response to thepolling command is received.

[0169] Next, another embodiment of the invention is described. While thenoncontact IC card 6 of the foregoing embodiment initiates theaforementioned detecting operation by reference to the timing ofdetecting the end of transmission of the polling command from theinformation read/write device 4, this may be modified as describedbelow.

[0170] Instead of transmitting the polling command, an informationread/write device 4 reduces electromagnetic wave output power for aspecific time period at specific time intervals (e.g., every 20 ms) asshown in FIG. 16(A). In addition, electromagnetic waves transmitted fromthe information read/write device 4 are not modulated except when it iscommunicating with a noncontact IC card 6.

[0171] Therefore, output voltage V of a power circuit 62 drops for aspecific time period at specific time intervals (e.g., every 10 ms) asshown in FIG. 16(B). The fixed time period during which the informationread/write device 4 decreases the electromagnetic wave output power is aperiod (e.g., 2 ms) which is sufficient for executing the aforementioneddetecting operation.

[0172] The noncontact IC card 6 of this embodiment may detect a risingedge of the output voltage V and perform the aforementioned detectingoperation by reference to the detected rising edge of the output voltageV. Alternatively, the noncontact IC card 6 may detect a falling edge ofthe output voltage V and perform the aforementioned detecting operationby reference to the detected falling edge of the output voltage V. Ineither case, the noncontact IC card 6 produces the same effects as thenoncontact IC card 6 of the foregoing embodiment.

[0173] The detecting operation performed by the noncontact IC card 6 ofthis embodiment is the same as that performed by the noncontact IC card6 of the foregoing embodiment, except that the timing of executing thedetecting operation differs.

[0174] As the information read/write device 4 does not transmit thepolling command in this embodiment, the noncontact IC card 6 and theinformation read/write device 4 communicate with each other bytransmitting a communication request command requesting communicationfrom the information read/write device 4 existing within thecommunicability area to the information read/write device 4. A specificcommunicability method is described below. Here, the method is describedusing an example in which two noncontact IC cards 6 are present withinthe communicability area.

[0175] The two noncontact IC cards 6 existing within the communicabilityarea individually transmit communication request commands requestingcommunication to the information read/write device 4. Thesecommunication request commands contain identification information whichidentifies the respective noncontact IC cards 6. Let us now consider acase where one of the two noncontact IC cards 6 transmits thecommunication request command at an earlier time.

[0176] When the information read/write device 4 receives thecommunication request command from one noncontact IC card 6 (hereinafterreferred to as the first noncontact IC card 6) existing within thecommunicability area, the information read/write device 4 determinesfrom which noncontact IC card 6 the communication request command hasbeen received. To begin noncontact communication with the firstnoncontact IC card 6 from which the communication request command hasbeen received, the information read/write device 4 transmits a commandfor establishing communication to the first noncontact IC card 6. Thiscommunication-establishing command contains the identificationinformation of the first noncontact IC card 6.

[0177] Upon receiving this communication-establishing command, the firstnoncontact IC card 6 transmits a response to thecommunication-establishing command back to the information read/writedevice 4.

[0178] When the response to the communication-establishing command isreceived from the first noncontact IC card 6, the information read/writedevice 4 transmits a read command, etc. to the first noncontact IC card6. The first noncontact IC card 6 then transmits a responsecorresponding to the received command back to the information read/writedevice 4. Noncontact communication between the information read/writedevice 4 and the first noncontact IC card 6 is carried out in thismanner.

[0179] When the noncontact communication with the noncontact IC card 6which has transmitted the communication request command is finished, theinformation read/write device 4 waits for a communication requestcommand to be transmitted from the second noncontact IC card 6 whilecarrying out the operation to reduce the electromagnetic wave outputpower for a specific time period at the specific time intervals. Whenthe communication request command is received from the second noncontactIC card 6, the information read/write device 4 transmits a command forestablishing communication to the second noncontact IC card 6 to beginnoncontact communication with the second noncontact IC card 6. Thiscommunication-establishing command contains the identificationinformation of the second noncontact IC card 6.

[0180] Upon receiving this communication-establishing command, thesecond noncontact IC card 6 transmits a response to thecommunication-establishing command to the information read/write device4.

[0181] When the response to the communication-establishing command isreceived from the second noncontact IC card 6, the informationread/write device 4 transmits a read command, etc. to the secondnoncontact IC card 6. The second noncontact IC card 6 then transmits aresponse corresponding to the received command back to the informationread/write device 4. Noncontact communication between the informationread/write device 4 and the second noncontact IC card 6 is carried outin this manner.

[0182] Communication between the information read/write device 4 and thenoncontact IC card 6 may be carried out by the following method as well.Here again, the method is described using an example in which twononcontact IC cards 6 are present within the communicability area.

[0183] When a plurality (two in this example) of noncontact IC cards 6exist within the communicability area, both noncontact IC cards 6transmit communication request commands requesting communication to thenoncontact IC card 6. These communication request commands containidentification information which identifies the respective noncontact ICcards 6. Let us now consider a case where one of the two noncontact ICcards 6 transmits the communication request command at an earlier time.

[0184] When the information read/write device 4 receives thecommunication request command from one noncontact IC card 6 (hereinafterreferred to as the first noncontact IC card 6), the informationread/write device 4 waits until the communication request command fromthe other noncontact IC card 6 (hereinafter referred to as the secondnoncontact IC card 6) for a preset time period. Here, the informationread/write device 4 receives the communication request command from thesecond noncontact IC card 6. The information read/write device 4transmits communication-establishing commands to the individualnoncontact IC cards 6. Upon receiving the communication-establishingcommand, the first noncontact IC card 6 transmits a response to thecommand back to the information read/write device 4. Upon receiving thecommunication-establishing command, the second noncontact IC card 6 alsotransmits a response to the command back to the information read/writedevice 4.

[0185] Upon receiving the responses to the communication-establishingcommands from the individual noncontact IC cards 6, the informationread/write device 4 transmits a read command, etc. to the firstnoncontact IC card 6. The first noncontact IC card 6 then transmits aresponse corresponding to the received command back to the informationread/write device 4. Noncontact communication between the informationread/write device 4 and the first noncontact IC card 6 is carried out inthis manner.

[0186] When the noncontact communication with the first noncontact ICcard 6 is finished, the information read/write device 4 transmits a readcommand, etc. to the second noncontact IC card 6. The second noncontactIC card 6 transmits a response corresponding to the received commandback to the information read/write device 4. Noncontact communicationbetween the information read/write device 4 and the second noncontact ICcard 6 is carried out in this way.

[0187] When the noncontact communication with the two noncontact ICcards 6 which have transmitted the communication request commands isfinished, the information read/write device 4 waits for a communicationrequest command to be transmitted from another noncontact IC card 6while carrying out the operation to reduce the electromagnetic waveoutput power for a specific time period at the specific time intervals(e.g., every 20 ms).

[0188] As thus far described, even when a plurality of noncontactcommunications media (e.g., noncontact IC cards) held close to eachother (overlapped state) are simultaneously brought close to acommunications apparatus, such as an information read/write device, itis possible to carry out noncontact communication with the individualmedia according to the present invention.

INDUSTRIAL APPLICABILITY

[0189] This invention applied to such equipment as identificationequipment, a personal authentication apparatus, an automatic ticketchecker, an automated teller machine for handling cash or electroniccash, and a charge collecting machine.

1. A noncontact communications medium comprising: a resonant circuitincluding: an antenna coil which generates an induced electromotiveforce and thereby transmits and receives signals by receivingelectromagnetic waves carrying information as a result of changes in theamount of electric power; and a capacitor parallel-connected to theantenna coil; a main circuit which processes a received signal obtainedby demodulating an output of said resonant circuit and outputs amodulated transmitting signal to said resonant circuit; and capacitancechange control means which varies the capacitance of said capacitoraccording to the amount of an output voltage occurring in said resonantcircuit and according to a specific change in said output voltage.
 2. Anoncontact communications medium comprising: a resonant circuitincluding: an antenna coil which generates an induced electromotiveforce and thereby transmits and receives signals by receivingelectromagnetic waves carrying information as a result of changes in theamount of electric power; and two capacitors parallel-connected to theantenna coil; a main circuit which processes a received signal obtainedby demodulating an output of said resonant circuit and outputs amodulated transmitting signal to said resonant circuit; a switch sectionwhich switches first one of said two capacitors between a connectedstate and a disconnected state; and a switch control section whichcontrols switching operation of said switch section according to theamount of an output voltage occurring in said resonant circuit andaccording to a specific change in said output voltage.
 3. The noncontactcommunications medium as recited in claim 2, wherein said switch controlsection controls the switching operation of said switch sectionaccording to a specific change in said output voltage when the outputvoltage occurring in said resonant circuit is equal to or higher than aspecific value.
 4. The noncontact communications medium as recited inclaim 2, wherein said switch control section controls the switchingoperation of said switch section according to a change at a rising edgeof said output voltage when the output voltage occurring in saidresonant circuit is equal to or higher than a specific value.
 5. Thenoncontact communications medium as recited in claim 2, wherein saidswitch control section controls the switching operation of said switchsection according to a change at a falling edge of said output voltagewhen the output voltage occurring in said resonant circuit is equal toor higher than a specific value.
 6. The noncontact communications mediumas recited in claim 2, wherein said switch control section judgeswhether the output voltage occurring in said resonant circuit becomeshigher when said first capacitor is in the connected state or in thedisconnected state when the switching operation of said switch sectionis controlled, and holds said switch section in the state in which saidoutput voltage becomes higher.
 7. The noncontact communications mediumas recited in claim 6, wherein said switch control section causes saidmain circuit to operate after holding said switch section.
 8. Thenoncontact communications medium as recited in claim 6, wherein saidswitch control section begins to supply electric power for operatingsaid main circuit after the amount of said output voltage has reached asufficient voltage for operating said main circuit under conditions inwhich said switch section is held.
 9. The noncontact communicationsmedium as recited in claim 2, wherein the capacitance of the secondcapacitor is such that, when the noncontact communications medium isoverlaid with another one with said first capacitor set in thedisconnected state in each noncontact communications medium, thefrequency of said electromagnetic waves and the resonant frequency ofthe two noncontact communications media approximately match each other.10. The noncontact communications medium as recited in claim 9, whereinthe capacitances of said two capacitors are such that, when said firstcapacitor is in the connected state, the frequency of saidelectromagnetic waves and the resonant frequency of said resonantcircuit approximately match each other.
 11. The noncontactcommunications medium as recited in claim 2, wherein the capacitance ofthe second capacitor is such that, when the noncontact communicationsmedium is overlaid with another one with said first capacitor set in thedisconnected state in each noncontact communications medium, theresonant frequency of the two noncontact communications media becomeshigher than the frequency of said electromagnetic waves.
 12. Thenoncontact communications medium as recited in claim 11, wherein thecapacitances of said two capacitors are such that, when said firstcapacitor is in the connected state, the frequency of saidelectromagnetic waves and the resonant frequency of said resonantcircuit approximately match each other.
 13. The noncontactcommunications medium as recited in claim 2, wherein said switch controlsection holds said first capacitor in the disconnected state in aninitial state.
 14. The noncontact communications medium as recited inclaim 2, wherein the noncontact communications medium is card-shaped inexternal appearance.
 15. A noncontact communications system comprising:a noncontact communications medium which comprises: a resonant circuitincluding: an antenna coil which generates an induced electromotiveforce and thereby transmits and receives signals by receivingelectromagnetic waves carrying information as a result of changes in theamount of electric power; and two capacitors parallel-connected to theantenna coil; a main circuit which processes a received signal obtainedby demodulating an output of said resonant circuit and outputs amodulated transmitting signal to said resonant circuit; a switch sectionwhich switches first one of said two capacitors between a connectedstate and a disconnected state; and a switch control section whichcontrols switching operation of said switch section according to theamount of an output voltage occurring in said resonant circuit andaccording to a specific change in said output voltage; and acommunications apparatus which communicates with said noncontactcommunications medium by radio.